The present invention provides a method and an apparatus for applying a coating to a surface using a directed vapor deposition (DVD) approach, and more particularly applying a directed coating to the line of sight region as well as non-line of sight and limited line of sight regions of a substrate.
The application of a coating to a substrate is required in a wide variety of engineering applications, including thermal or environmental protection, improved wear resistance, altered optical or electronic properties, decorative, biocompatibility, etc. In each of these cases, the ability to deposit compositionally controlled coatings efficiently, uniformly, at a high rate, with a high part throughput, and in a cost effective manner is highly desired
As stated above, the ability to uniformly deposit ceramic or metallic coatings onto substrates is desirable for a number of applications. They include the deposition of metal on fibers to create metal matrix composites, deposition of coatings having low shear resistance and good thermochemical stability on the fibers used in ceramic matrix composites and the deposition of metals on sacrificial fiber templates to create hollow fibers. More generally, vapor deposition approaches which allow the creation of conformal coatings on a variety of non-planar substrates is also of interest. For example, the deposition of reaction inhibiting coatings onto the surfaces of a multi-airfoil vane segment for a gas turbine engine.
Several options for creating coatings of this type exist. These include chemical vapor deposition (CVD), electroplating processes and physical vapor deposition (PVD) approaches (such as electron beam evaporation or inverted cylindrical magnetron sputtering). However, despite the many needs, the advancement of processing approaches for these applications above are limited by several factors; namely, the inability to uniformly coat such substrates without sophisticated substrate translation and rotation capabilities, the inability to deposit metal, alloys and ceramics with the same process, the inability to create a coating with the desired microstructure, and low deposition rates which often limit high volume throughputs.
In CVD, uniform coating thicknesses are readily produced in some cases. However, the deposition rates can be low and the process often requires the use of toxic (and expensive) precursor materials. The deposition of multicomponent alloys can also be challenging. Electroplating can provide uniform coating over the surface of complex shaped parts. Although useful for depositing elemental layers, this process is less suitable for the creation of alloy or ceramic coatings.
In PVD approaches vapor atoms are created in high vacuum and deposited onto a substrate. One method to created vapor atoms is sputtering. A wide variety of materials can be deposited, but deposition rates are low. The high vacuums employed in these techniques result in few collisions with the background gas resulting in “line-of-sight” coating. Thus, substrate manipulation and/or shadowing is required to achieve acceptable coating uniformity on non-planar surfaces. Higher deposition rates require more energetic/higher density plasma sputtering (e.g. magnetrons).
Atomic vapor can be more rapidly created using electron beam evaporation approaches. However, the materials utilization efficiency (MUE) of electron beam physical vapor deposition is often low. When a relatively long source to substrate distances is required, the deposition efficiency can be low and the deposition rate limited. The high vacuum environments required for the creation of electron beam also lead to line of sight coating.
Electron beam-physical vapor deposition (EB-PVD) is a widely used method for the high-rate production of atomic and molecular vapor (metal or ceramic) for vapor deposition of a coating. During EB-PVD, vapor is transported to a substrate under high vacuum conditions where it condenses on surfaces that are in the line-of-sight of the vapor cloud source. This requires the use of complicated translation and rotation systems and shadowing to deposit a uniform coating onto complex or non linear structures that contain areas not in line of sight of the vapor stream. Even with known methods and equipment, EB-PVD processes often fail to create uniform coating thicknesses on difficult to coat locations of a substrate, i.e., non-line of sight and limited line of sight areas.
Electron beam—physical vapor deposition (EB-PVD) of metal and ceramic coatings can be quite costly to apply due to high equipment cost, low deposition efficiencies and relatively low deposition rates. The high equipment costs of EB-PVD are a result of the high vacuum environment, which is necessary during deposition, the high cost of high power electron beam guns, and the sophisticated component manipulation systems needed to achieve uniform coating on non-planar substrates. The operating pressure defines the vacuum pump requirements with lower pressures generally needing more expensive pumps. The low deposition rate and low materials utilization efficiency (MUE) of EB-PVD is related to the distribution of vapor cloud as it leaves the evaporated source. When relatively long source-to-substrate distances are required, the deposition efficiency is dramatically reduced.
Low deposition efficiencies result from cloud spreading beyond the periphery of the substrate and non line-of-sight deposition. One approach to reduce the spread of the cloud exploits entrainment of the vapor on a controllable inert (e.g. helium or argon) carrier gas flow. Such an approach is used in electron beam—directed vapor deposition (EB-DVD). In this approach, the combination of a continuously operating electron beam gun (modified to function in a low vacuum environment) and an inert carrier gas jet. In this system, the vapor plume is intersected with a rarefied trans- or supersonic inert gas jet, to entrain the evaporated cloud in a non-reacting gas flow and transport it to a substrate under low vacuum conditions. Deposition of the atomistic cloud then occurs by gas phase scattering from the streamlines of the flow and is deposited onto the substrate at high rates and with high materials utilization efficiency. However, this process may still result in unacceptable non-uniform deposition of coatings, especially when comparing line of sight to non line of sight areas. In addition, current systems of this type require the use of large amounts of gas, which results in substantial costs associated with the procurement of the gas as well as costs associated with the pumping capacity.